Which enemy do you think is the most powerful that the Strawhats have met?
Is the enemy you choose different from the enemy which you think is most difficult to battle?
Personally, I think the most powerful enemy the Luffy's nakama have met is Aokiji.
The reason, other than he's cool, is that he's a DF user that has no fear of the sea.
He rides his bike on the ocean to relieve stress!
Everyother DF user doesn't have enough power to move across the ocean solely on their own powers:
Eneru rides a self-sustaining ship that he created after years of other mortals' labor and excavating gold.
If he were to create the ship on his own and to power it completely (the ship has an engine, and Eneru just assists its ignition) then he'd be either on par with or stronger than Aokiji.
On the topic of how Eneru travels, he and his priests just showed up mysteriously from nowhere.
It never says how they got to skypiea, but I doubt Eneru carried his priests and enforcers.
(how can they ride his lightning? Eneru's own enforcers died from collateral damage when Eneru took out Kamakiri)
Crocodile, well, he's afraid of water, so flying in the air would be his obvious mode of transport if not by ship.
However, does he have enough stamina to sustain flotation across islands?
Admittedly, he is a powerful man, but he has never conveyed that he could stay in logia form and fly around for days.
In addition to the stamina problem, unlike Aokiji, Crocodile can't defeat the ocean with his power capacity.
If he were anywhere near as strong as Aokiji, he could dry the sea and control the weather without assistance.
Crocodile was able to control the climate of Alabasta using rain dance powder.
Captain Smoker is only semi-formidable compared to Crocodile.
He can turn into smoke and capture criminals (and he can capture Luffy really easily) but he hasn't anywhere near enough power to be a force of nature, unlike Kiji, Enel, and Croc.
Now the other non-logia powered users (except Hawkeyes Mihawk, he gets his own analysis)
Buggy's afraid of his own buggy bomb, and lacks the strength to break out of the ropes that Nami tied him in, near the beginning.
Obviously, he's going to be stronger now that he's experiencing the Grand Line, but let's wait and see how he turns out.
Alvida is invincible to physical attacks, but she seems to lack offensive strength compared to Luffy's crew.
Even Nami is stronger, seriously.
Again, let's see how she turns out later on.
Captain Kuro lacks the superhuman strength of the later enemies, but he was challenging for the time he fought Luffy.
Don Kreig relies on weapons and armor he's collected in his experience, although he is very strong physically, but he definitely pales in comparison to the godly Mihawk.
Arlong is definitely superhuman level, but even with his mighty merman strength and devious wit, he's small fry () compared to others.
Doflamingo and Kuma get their separate sections too, since they're shichibukai.
Blackbeard and his crew seems like a deadly force on par with shichibukai, so I'll talk about them later.
Foxy was very very tricky (to Luffy), but he isn't very powerful. Luffy basically took him out with one punch when he had the chance, but Foxy used every weapon in his arsenal and it exhausted Luffy, but Luffy's will is stronger than Foxy's so he wins. (it's shounen, it can't be helped)
Now the 7 pirate gods level.
Mihawk to me seems to be the strongest, or one of the strongest, of the sichibukai.
His sword waves just rip through other ships as if they were paper.
In addition, he's nearly self-sufficient on the seas, since he rides a raft, without sails, on the Grand Line, back and forth, in his sleep!!!
This guy is #2 in my opinion.
Don Quixote Doflamingo is a scary man with the ability to control another person's actions despite what their mind tells them.
Luffy's gonna suffer when faced with him.
I don't know much about Bartholomew Kuma, but his former bounty was near Doflamingo's, so I guess he's pretty powerful.
Blackbeard and his crew boast power in addition to body size, despite their small raft that they use as a ship.
Van Auge was able to snipe seagulls from miles away.
He's obviously going to fight Sogeking.
Doc is able to make apples explode when eaten.
Gonna fight Chopper I guess.
Laffite seems to be a stylish diplomat, who's strong for some reason.
Probably fights against Sanji.
Champion was really loud, and supposedly has great strength, but we've seem nothing but loudness from him.
Blackbeard is the captain and is capable of smashing heads with arm strength alone, but we haven't actually seen any powers from him.
Now Ace isn't an enemy, but he may fight Luffy again some time later.
He's really strong, but it's situational.
When he's confident, he's probably as strong as Aokiji.
When he finds himself being the one at err, he can be taken down by simple villagers.
How strong is this guy on the high seas?
Well he rides a really cool powerboat that runs on his fire across the Grand Line, so he's got a lot of stamina.
However, the only firepower we've seen from this guy is against the Billions of Baroque Works (it's not really billions of people!), and he beat them with neat little flippy tricks and some fire, obviously not taking them seriously.
Future fights may be against Shanks and Whitebeard, but we have no clue as to how powerful these guys are.
Well, except Shanks seemed to have battled Mihawk in a swordfight with one hand... and his newbie crewmate has a bounty of 97 mil...
Now why am I doing this?
Because I'm bored and have insomnia. Good night peoples.